1. Field
The invention relates to a telephone system which allows voice communication using an Internet Protocol (IP) network and, for example, a signaling protocol such as Session Initiation Protocol (SIP) or Media Gateway Control (Megaco). More particularly, the invention relates to improvement of a system which forms a session between a terminal and server using Transmission Control Protocol (TCP) or User Datagram Protocol (UDP).
2. Description of the Related Art
In recent years, telephone systems which transmit voice packets via an IP network have attracted attention. A technical background will be explained below taking a typical SIP telephone system as an example. A SIP telephone system includes a server (SIP server) used to manage the addresses, extension numbers, and the like of telephone terminals (to be referred to as IP terminals hereinafter). Each IP terminal forms a session with the SIP server using UDP or TCP, but the protocol to be used is fixedly decided at the time of registration of that terminal in the server. That is, re-registration procedures of a terminal are required to switch the connection using UDP to that using TCP or vice versa, and the connection is disconnected upon deregistration.
TCP imposes a heavier load on the system than UDP. Hence, the number of sessions using TCP connections is limited, and a session over the limit is formed using a UDP connection. However, there are services unique to TCP, and an encrypted protocol such as Transport Layer Security (TLS) is not available for UDP. Hence, in order to conduct encrypted communication from a UDP-connected terminal, procedures in which that terminal waits for an empty TCP session after the connection is canceled, and requests re-connection are required. If the user is busy, disconnection results. Hence, some measures are demanded.
Jpn. Pat. Appln. KOKAI Publication No. 2001-298479 discloses the following technique. That is, in an Internet telephone system, if the network is not busy, UDP is used. However, if the network becomes busy, TCP is used to prevent interrupted speech. This is a technique that uses TCP or UDP in voice packet transmission, and cannot be applied intact to registration of a terminal in a server.
Jpn. Pat. Appln. KOKAI Publication No. 2004-32039 discloses a network facsimile apparatus, which detects a packet delay, and conducts UDP communication if the delay is large or TCP communication if the delay is small. However, this technique cannot be applied to registration of a terminal in a server.
Jpn. Pat. Appln. KOKAI Publication No. 2002-281103 discloses a technique which switches TCP and UDP according to the occupation state of a reception buffer on the client side in a method of transferring media data from a server. However, this technique cannot be applied to registration of a terminal in a server.
As described above, in the existing telephone systems, the protocol used to form a session is fixedly decided upon registering an IP terminal in a server. Hence, in order to switch the protocol, re-registration is required after deregistration, and a trouble such as disconnection of voice communication may be posed. In addition, a problem that impairs easy encrypted communication is posed, and some measures are demanded.